The present invention relates to the sealing of anodized aluminum and aluminum alloy structures to achieve superior dielectric properties. More particu1arly, the invention relates to the production of hard, abrasion resistant dielectric members and to electrostatic imaging processes and apparatus utilizing such members.
Electrostatic printers have been proposed which make use of a member commonly in the form of a cylinder and consisting of an electrically conductive core coated with a dielectric material capable of receiving a pattern of electrostatic charge from a discharge device. This device is so controlled that a selected pattern of charge can be applied to the surface of the cylinder as it passes the device. Subsequently, this pattern is toned using, for example, particulate toner supplied by a suitable feed system, and then the toned image on the cylinder is transferred at a nip with a pressure roller to a receptor medium such as a sheet of paper as the paper passes through the nip. This transfer may or may not include toner fusing depending upon the nip pressure and also, for best results, on whether or not the cylinder and roller are skewed relative to one another. Subsequently, any remaining toner is scraped off mechanically and any electrostatic charge on the cylinder is dissipated as the cylinder passes a discharge device prior to receiving another selected pattern of charge. Apparatus of this type is disclosed in commonly assigned U.S. Pat. No. 4,267,556.
In such a printer, the cylinder must satisfy a number of design criteria. Firstly, the surface should receive the desired pattern of charge accurately and without variations in electrostatic intensity within the pattern. The surface should maintain the pattern without significant dissipation before reaching the nip, and also the pattern must be dissipated by the discharge device leaving as nearly as possible no charge pattern on the cylinder. A11 of these criteria should be met ideally in a range of temperature and humidity variations which may be controlled within limits. Other desirable criteria relate to the mechanical requirements of the cylinder surface. The forces applied at the nip demand that the dielectric surface withstand a large distributed load which will, of course, result in some strain on the cylinder. Further, because the paper feeding into and out of the nip represents an impact loading and unloading, there are suddenly-applied local forces which the dielectric layer must resist. Also, when the cylinder and pressure roller are skewed, the paper is made to follow the pressure roller rather than the cylinder to cause sheer in the toner. The resultant relative movement between the dielectric layer and the paper could result in abrasion of the dielectric layer because the toner acts as an abrasive between the paper and the surface of the layer. The layer must withstand a mechanical scraper normally used to strip excess toner off the cylinder after the majority of the toner has been transferred to the paper. Other potential problems relate to nonuse of the machine while a load is maintained at the nip, and also to ambient temperature and moisture variations, which should have no significant lasting effect on the cylinder.
U.S. Pat. No. 4,195,927 discloses electrophotographic apparatus identical in construction to the '556 printing apparatus, except for the means for forming the latent electrostatic image on the dielectric cylinder. In the '927 apparatus, the latent electrostatic image is formed on a photoreceptor by conventional electrophotographic techniques, and transferred by TESI to the dielectric cylinder. The criteria for the '927 dielectric cylinder match those discussed above.
Hardcoat anodization of aluminum and aluminum alloys is an electrolytic process which is used to produce thick oxide coatings with substantial hardness. Such coatings are to be distinguished from natural films of oxide which are normally present on aluminum surfaces, and from thin, electrolytically formed barrier coatings. The anodization of aluminum to form thick dielectric coatings takes place in an electrolytic bath containing an acid, such as sulfuric or oxalic acid, in which aluminum oxide is slightly soluble. The production techniques, properties, and applications of these aluminum oxide coatings are described in detail in The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick and R. Pinner, fourth edition, 1972, published by Robert Draper Ltd., Paddington, England (chapter IX page 563). Such coatings are extremely hard and mechanically superior to uncoated aluminum. However, the coatings contain pores in the form of fine tubes with a porosity on the order of 10.sup.10 to 10.sup.12 pores per square inch. Typical porosities range from 10 to 30 percent by volume. These pores extend through the coating to a very thin barrier layer of aluminum oxide, typically 300 to 800 Angstroms.
For improved mechanical properties as well as to prevent staining, it is customary practice to seal the pores. One standard sealing technique involves partially hydrating the oxide through immersion in boiling water, usually containing certain nickel salts, which form an expanded boehmite structure at the mouths of the pores. Oxide sealing in this manner will not support an electrostatic charge due to the ionic conductivity of moisture trapped in the pores.
Another method of sealing an anodized aluminum member is disclosed by Quaintance in U.S. Pat. No. 3,715,211. This is a method of cold sealing by the photopolymerization of an organic liquid applied to the anodized surface.
U.S. Pat. No. 3,615,405 discloses a method of fabricating an electrophotographic oxide surface by means of impregnating the porous oxide surface of an aluminum article with an "imaging material." The process creates a member with direct contact between the imaging material and the conductive substrate over which the porous oxide layer is formed. This patent does not disclose a step of dehydrating the oxide pores prior to impregnation with an imaging material (the article is placed in a vacuum oven only after coating with an impregnant material). As such, there is a likelihood of trapped moisture, which would be deleterious to the dielectric properties of the impregnated anodic layer. In order to provide discharge in radiation struck areas, U.S. Pat. No. 3,615,405 requires contact of the "electrographic imaging material" with the conducting substrate. In the present invention, the sealing material contacts an insulating barrier layer.
These foregoing references cannot be used for the processing of an aluminum cylinder for use in electrostatic imaging with pressure fusing and transfer as discussed above and in U.S. Pat. No. 3,662,395. Table 2 of that patent indicates that a porous aluminum oxide surface sealed with teflon is not satisfactory for electrostatic imaging due to the low breakdown voltage and low pore insulation resistance of the aluminum oxide surface. The organic resin sealant fails to achieve the necessary high abrasion resistance and coating hardness.
A drum coated with an insulating film capable of supporting an electrostatic charge is disclosed in U.S. Pat. No. 3,907,560. The dielectric surface is a barrier layer aluminum oxide film since it is stated that the porous anodized aluminum oxide layer functions as a conductor rather than a dielectric. Although a barrier layer anodized aluminum film is a good insulator, being non-porous, the maximum thickness of barrier layer films is restricted to the region of at most 1/2 to 1 microns. At this thickness, the maximum voltage the layer will support is limited and the surface is not hard in a conventional sense since any localized strains are transmitted through the thin film with subsequent deformation of the aluminum substrate.
The limitations of the thin barrier film are overcome in U.S. Pat. Nos. 3,937,571 and 3,940,270 by the use of a duplex anodized aluminum coating. The coating is prepared by electrolytically oxidizing an aluminum surface and thereafter continuing the electrolytic oxidization under conditions which produce a barrier aluminum oxide layer. Not only does this increase the complexity of fabricating the anodized layer, but the limiting thickness is approximately 20 microns and the surface potential to which the oxide layer may be charged has a maximum of 620 volts.
Commonly assigned U.S. patent application Ser. No. 072,524, which is a continuation-in-part of application Ser. No. 822,865, now abandoned, discloses a method for forming a dielectric surface layer involving the preliminary dehydration of an anodized aluminum member followed by impregnation of surface apertures of the dehydrated member with an organic dielectric material. The preliminary dehydration may be accomplished by heating the anodized member in a vacuum or in air, or alternatively by storing it in a desicant container. This application discloses a class of impregnant materials broadly described as organic resins. The method disclosed therein has been found effective to fabricate a dielectric surface with improved resistivity, dielectric properties, and toner release properties. It has been observed, however, that the dielectric properties are deleteriously affected by elevated humidities. Because these materials are usually applied at room temperature, special measures must be taken to control the environment during impregnation to minimize the risk of dehydration. Furthermore, it can be difficult to remedy the problem of an initially uneven application of the impregnant material.
Commonly assigned U.S. patent application Ser. No. 346,346, which is a continuation-in-part of Ser. No. 164,482, which is a continuation-in-part of Ser. No. 155,354 filed June 2, 1980, discloses an improvement to the above method wherein the impregnant materials are metallic salts of fatty acids. These are typically applied to seal the anodized aluminum member while the latter is maintained at an elevated temperature above the melting point of the impregnant material. These materials provide the advantages of ease of fabrication and improved dielectric properties at high humidities, but may suffer undesirably high dielectric absorption under certain conditions (such as prolonged storage in high humidities). In other words, under unfavorable operating conditions there will be a tendency toward retention of subsurface charge in the impregnated anodic layer. During neutralization of the dielectric surface this charge will migrate to the surface providing an undesirable residual potential.
U.S. Pat. No. 3,782,997 discloses a method for treating anodized beryllium members to produce corrosion resistant dielectric surfaces. After anodizing, the beryllium members are cleaned, baked at 250.degree. F. in a normal atmosphere, then at 200.degree. F. in a vacuum to remove residual moisture. The article is cooled at 160.degree. F. to seal the pores with an epoxy resin or similar material, using high pressure to facilitate impregnation. Excess material is removed by bleeding the member or rinsing it with a solvent. Finally, the member may be maintained at 212.degree. F. for several hours to cure the impregnant material. This reference does not teach the production of a dielectric member having the surface properties required for good toner transfer under pressure. The method and product of this reference suffer some of the same disadvantages as cited above for Ser. No. 072,524.
Accordingly, it is a primary object of this invention to provide desired dielectric properties in the treatment of members of porous anodized aluminum and aluminum-based alloys. A re1ated object is to improve the dielectric strength and increase the resistivity of such members. Another related object is the achievement of thick dielectric surface layers with a high voltage acceptance and low charge decay rates.
It is a further object of the invention to provide a treated aluminum surface that will yield essentially total pressure transfer of a toned electrostatic image to plain paper and other substrates.
Yet another object of the invention is the achievement of a surface which maintains the above properties at elevated humidities.
Still another object of the invention is that the fabrication technique be easily implementable. As a related object, the technique should allow simple remedial steps to meet the above criteria where the initial fabrication is unsuccessful.
Further objects of the invention are hardness and abrasion resistance which would allow pressure transfer and fusing of electrostatic toner, while providing an extended operating life.
It is also desirable that such surfaces permit neutralization of most or all of any residual electrostatic image, i.e. minimal dielectric absorption.